1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser beam printer, an electrostatic recording apparatus and the like, and more particularly, it relates to an image forming apparatus in which multi-color printing can be performed.
2. Related Background Art
In recent image forming techniques, different color visualized images or different information images have been composed on a single sheet, and, to this end, image forming apparatuses having a plurality of developing devices have been in the market. Among these techniques, a technique in which while a latent image bearing member is being rotated by one revolution latent images are visualized with plural colors by using two or more developing devices and at the same time the visualized images are transferred onto a sheet has been proposed. For example, U.S. Pat. Nos. 4,572,651 and 4,416,533 disclose a technique in which development is performed by applying DC bias to two developing device to generate constant electric fields. These Patents mainly disclose a method for forming a latent image but do not teach problems during the development.
On the other hand, U.S. Pat. No. 4,349,268 and Japanese Patent Application Laid-Open No. 56-144452 disclose a technique in which second color development is performed by using AC developing bias with non-contact development, and Japanese Patent Application Laid-Open No. 56-12650 discloses a technique in which non-contact development is performed by using DC bias. These techniques aim to prevent second color developer from rubbing a first color developed image, thereby preventing distortion of the image. Incidentally, the Japanese Patent Application Laid-Open No. 56-144452 does not teach or disclose potential of the first color developed image at all.
As mentioned above, in the conventional image forming apparatuses for forming a plurality of developed images, the technique in which second development is effected not to distort the previously developed image is used.
Similarly, U.S. Pat. No. 4,660,961 discloses a technique in which potential of a latent image to be firstly developed is increased (level-up). In this technique, after a first color developed image is formed, by uniformly charging an entire surface of a latent image bearing member with polarity same as that of developer, the potential of the latent image to be firstly developed can be substantially the same as potential of a non-image portion, so that distortion of the firstly formed image can effectively be prevented. An image forming apparatus using this technique is called as a one-pass multi-color print image forming apparatus of negative-negative re-charging type which has recently been investigated widely.
Further, recently, particularly in a field of high speed image forming apparatuses, a multi-color image forming apparatus in which amorphous silicone having high durability is used in a photosensitive member has been studied.
However, in the above-mentioned conventional techniques, there arises the following problem.
In some photosensitive members, since electrostatic capacity thereof becomes great, sufficient charging cannot be achieved by the re-charging, and, due to limitation regarding compactness and cheapness of the image forming apparatus, a re-charger having sufficient ability cannot be used. Thus, also in the above-mentioned negative-negative re-charging process, in dependence upon the construction of the photosensitive member and the image forming apparatus, since the potential of the latent image to be firstly developed cannot be substantially the same as the potential of the non-image portion, the first color developed image is distorted during the second color development, or density unevenness or fogging occurs during the second development.
FIGS. 12A to 12F show steps of an example of a conventional image forming process. In each of FIGS. 12A to 12F, surface potential of a photosensitive member is schematically shown.
In FIG. 12A, the photosensitive member is charged by a corona charger to +400V, for example. Then, in FIG. 12B, first exposure is effected regarding the photosensitive member on the basis of a first image signal to reduce surface potential of an exposure portion to +50V (at the maximum), for example, thereby forming a first electrostatic latent image. Then, in FIG. 12C, the exposure portion is inversion-developed by applying developing bias voltage (for example, +300V: shown by the dotted line) to a sleeve of a first developing device.
After first development, in FIG. 12D, re-charging is performed. In this case, as mentioned above, in dependence upon the construction of the photosensitive member and the image forming apparatus, the first developing image portion cannot be charged sufficiently while suppressing increase in potential of the first developing non-image porion. For example, the first developed image portion is charged merely to 250V, for example. Thus, if, in FIG. 12E, exposure responding to second image information is performed, and, in FIG. 12F, inversion-development is performed by applying developing bias voltage (for example, +300V: shown by the dotted line) to a sleeve of a second developing device, second developer will be developed also at the first developing image portion, thereby causing color-mixing.
If the bias voltage of the second developing device is selected to, for example, +200V to prevent the color-mixing, sufficient second developing density cannot be obtained, and, since a difference between the developing bias and the potential of the first developing non-image area also becomes great (200V), reversal toner having polarity opposite to desired polarity will be developed. Even the toner having desired polarity, unless the potential of the photosensitive member is greater than the developing bias potential by about 100V, the toner is developed. Thus, in this example, it is desirable that the potential of the second developing non-image portion becomes 400V greater than the developing bias by 100V at the first developing portion and the first non-developing portion. That is to say, after re-charging, the surface potential of the drum at the second development position must be converged regarding the first developing non-image portion and the first developing image portion.
As a means for solving the above problem, it is considered that the construction and bias of the re-charger are selected appropriately. FIG. 13 is a graph showing test results regarding relations between current applied to a corotron wire of the re-charger and drum surface potentials at the first developing non-image portion and the first developing image portion after the re-charging. From FIG. 13, it can be seen that it is difficult to converge the first developing non-image portion and the first developing image portion by the corotron re-charger (this is referred to as "re-charge convergence" hereinafter).
Alternatively, it is considered that a scorotron re-charger is used as a re-charger so that drum direction current flowing through the first developing non-image portion is reduced and drum direction current flowing through the first developing image portion is maintained. FIG. 14 is a graph showing test results regarding relations between voltage applied to a grid and drum surface potentials at the first developing non-image portion and the first developing image portion after the re-charging. As can be seen from FIG. 14, when the voltage applied to the grid becomes substantially the same as the first developing non-image portion, the first developing image portion can be sufficiently charged while suppressing the potential of the first developing non-image portion, thereby improving the convergence.
However, in such grid voltage, since great drum direction current is required for sufficiently increasing the potential of the first developing image portion after re-charging, a larger-sized re-charger must be provided, resulting in disadvantage regarding the cost and compactness (inner space) of the apparatus. This is true particularly when an amorphous silicone drum having great specific inductive capacity (hard to be charged) is used.
As a further method, there has been proposed a technique in which, after first development, uniform exposure is effected in a process prior to second exposure, and potential of a latent image for a first color developed image is made substantially the same as a non-development portion. However, in this technique, since a uniform exposure means must be provided for the process prior to the second exposure, disadvantage regarding the cost and compactness of the apparatus is arisen.